dwiextract: Fix extraction based on TRT
dwiextract: Fix extraction based on TRT

Fix to selection of DWI volumes where there are volumes with the same phase encoding direction but different readout times.

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3